The present invention relates generally to electrical microcircuit structures incorporating insulating or passivating layers of phosphosilicate glass. More particularly, the invention concerns a method for reducing the corrosive effect of such layers on circuit metallization.
Slica-based glasses are widely used in the manufacture of monolithic microcircuits, both to passivate the surfaces of semiconductor substrates and to provide insulation for thin-film components and metallization layers. Various techniques are used to deposit the glasses. For example, semiconductor circuits commonly are passivated by chemical vapor deposition (CVD) of a glassy material, a method that permits high-temperature glasses to be deposited at relatively low temperatures. Another frequently used method is based on the application of a siliceous material in solution form, followed by a heat exposure to provide a hardened glassy layer. The solution can be applied by spraying or dipping, but spinning is the usual method, the resulting films thus being referred to as "spun-on-glass" (SOG). Conformal coatings of controlled composition may be produced by fusing finely ground glass particles at or slightly below the glass' softening point. The fusion flows the glass, forming an adherent bond to the underlying circuit substrate.
Phosphorus is often added to the silica glasses used in microcircuit applications. The additive functions to reduce intrinsic stresses in the deposited layers and thereby prevent crack formation, to act as a getter for alkaline ions, or (in somewhat higher concentrations) to serve as a diffusion source. Despite these benefits, the presence of phosphorus in microcircuit passivation layers has been found to promote corrosion of adjoining aluminum metallization. It is known that phosphorus-doped silica glass layers absorb moisture in an amount proportional to the P.sub.2 O.sub.5 content of the glass. Phosphorus oxides readily react with water, forming strong acids that will attack exposed metals.
Previous efforts to eliminate the corrosion potential of phosphosilicate glass layers have been directed at preventing moisture from coming into contact with them. One such method is to package the entire microcircuit in a hermetically sealed enclosure. Another is to cap the phosphosilicate glass with a moisture impermeable layer, typically a phosphorus-free glass. Although such procedures are effective, they share the disadvantage of being relatively high cost solutions to the problem.
A primary object of the present invention thus is to provide a way to eliminate the corrosion potential of phosphosilicate glass layers in an economical and effective manner.
Another, more specific object of the invention is to provide a method for reducing the phosphorus concentration in exposed surface regions of a phosphosilicate glass layer thereby to reduce the corrosion potential of the layer.
A further, still more specific object of the invention is to reduce the phosphorus concentration in exposed surface areas of a phosphosilicate glass layer by exposing those areas to a humid atmosphere for a time sufficient to hydrate a substantial portion of the available phosphorus in a region adjoining the surface, then removing the resultant acidic hydration products by rinsing the surface with water.